In essence, Facebook users didn’t think they wanted constant, up-to-the-minute updates on what other people are doing. Yet when they experienced this sort of omnipresent knowledge, they found it intriguing and addictive. Why?

Social scientists have a name for this sort of incessant online contact. They call it “ambient awareness.” It is, they say, very much like being physically near someone and picking up on his mood through the little things he does — body language, sighs, stray comments — out of the corner of your eye. Facebook is no longer alone in offering this sort of interaction online. In the last year, there has been a boom in tools for “microblogging”: posting frequent tiny updates on what you’re doing. The phenomenon is quite different from what we normally think of as blogging, because a blog post is usually a written piece, sometimes quite long: a statement of opinion, a story, an analysis. But these new updates are something different. They’re far shorter, far more frequent and less carefully considered. One of the most popular new tools is Twitter, a Web site and messaging service that allows its two-million-plus users to broadcast to their friends haiku-length updates — limited to 140 characters, as brief as a mobile-phone text message — on what they’re doing. There are other services for reporting where you’re traveling (Dopplr) or for quickly tossing online a stream of the pictures, videos or Web sites you’re looking at (Tumblr). And there are even tools that give your location. When the new iPhone, with built-in tracking, was introduced in July, one million people began using Loopt, a piece of software that automatically tells all your friends exactly where you are.

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This is the paradox of ambient awareness. Each little update — each individual bit of social information — is insignificant on its own, even supremely mundane. But taken together, over time, the little snippets coalesce into a surprisingly sophisticated portrait of your friends’ and family members’ lives, like thousands of dots making a pointillist painting. This was never before possible, because in the real world, no friend would bother to call you up and detail the sandwiches she was eating. The ambient information becomes like “a type of E.S.P.,” as Haley described it to me, an invisible dimension floating over everyday life.

“It’s like I can distantly read everyone’s mind,” Haley went on to say. “I love that. I feel like I’m getting to something raw about my friends. It’s like I’ve got this heads-up display for them.” It can also lead to more real-life contact, because when one member of Haley’s group decides to go out to a bar or see a band and Twitters about his plans, the others see it, and some decide to drop by — ad hoc, self-organizing socializing. And when they do socialize face to face, it feels oddly as if they’ve never actually been apart. They don’t need to ask, “So, what have you been up to?” because they already know. Instead, they’ll begin discussing something that one of the friends Twittered that afternoon, as if picking up a conversation in the middle.

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You could also regard the growing popularity of online awareness as a reaction to social isolation, the modern American disconnectedness that Robert Putnam explored in his book “Bowling Alone.” The mobile workforce requires people to travel more frequently for work, leaving friends and family behind, and members of the growing army of the self-employed often spend their days in solitude. Ambient intimacy becomes a way to “feel less alone,” as more than one Facebook and Twitter user told me.